IMO If I had to take it to the next level for the big block I'd at least add the rear torque boxes..You've got the twisting aspect of the equation taken care of now it's time to make sure the torque generated from the big block doesn't pull on the front spring mounts...They were there from the factory for a reason...
Is it an absolute necessity ?? Possibly not..But you've already done the hard modification the rear torque boxes would be an easy addition and good piece of mind eliminating a potential problem ...